Electron gun for traveling wave tube



Jan. 7, 1958 G. R. BREWER ELECTRON GUN FOR TRAVELING WAVE TUBE 2 Sheets-Sheet 1 Filed Dec. 2, 1954 G. R. BREWER ELECTRON GUN FOR TRAVELING WAVE TUBE Jan. 7, 1958 2 Sheets-Sheet Filed Dec. 2, 1954 'United States Patent ELECTRON GUN FOR TRAVELING WAVE TUBE' George R. Brewer, Palos Verdes Estates, Calif., assignor to Hughes Aircraft Company, a corporation of Delaware Application December 2, 1954, Serial No. 472,581 y 1 Claim. (Cl. 315-35) This invention relates to electron discharge devices and more particularly to an electron. gunlfor producing an electron stream having 'a substantially uniform diameter.

In electron stream-type of traveling-wavetubesfan electron stream is projected through a conductive helix to interact with land amplifya traveling wave propagated therealong. A solenoid is' generally disposed concentrically labout the helix to focus or lconfine the 'I stream electrons. The stream is ysaid to be focused in Brillouin flow when the cathode from which thek stream electrons are emitted is not immersed in the Vmagnetic field developed by thefocusing solenoid. The stream is said to be in confined ow when the cathode is immersed in a uniform magnetic field. Both of theseltypes of electron flow may be usefully employed. However, Brillouin flow is somewhat more difficult to produce due to the particular type of stream focusing structure employed in developing such iiow.` lFurther, in either case original disturbances in thevelectron streaml cause thestream to ripple or to vary in diameter. Travelingwave axial electric fields decay very rapidly with vstream radius.l

Rippling thus can and does substantially decrease the power and efficiency of a traveling-Wave tube by periodically preventing optimum traveling-wave interaction.

It is therefore an object of the invention to provide 'an electron gun for reducing the diameter variations in a magnetically confined electron stream.

, it is another. object of the invention to provide an improved electron gun for 'a traveling-wave tube in lwhich electron flow is magnetically confined. f

In accordance with the invention a cylindrical cathode, which may yhave a substantiallyflat emission surface, is employed as an electron source. A focusing electrode is positioned'about vthe cathode and maintained at Ian `appropriate potential to develop a `converging electron stream. The stream is then directed through an annular disc electrode` at such an angle that the rate of change of the stream radius with respect to distance is substantially zero at a point in a subsequent equipotential region. The total laxial magnetic flux linking the stream is mean while maintained constant in order to preserve a true confined electron flow.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understoodfrom the followingv description considered in connection with the accompanying drawings, in which an embodiment ofthe invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description in llypanct4 lare not intended as a definition of the limits of the invention.

Fig. 1 is a sectional view of atraveling-wave tube with ice effect of entrance conditions upon the ripples of an electron stream; and

Fig. 3 is a diagrammatic sectional view of a solid electron stream showing the trajectories of the outer electrons which may be produced with the use of the gun of the present invention.

Referring to the drawing, there is illustrated in Fig. l a traveling-wave tube 10 having an input matching cavity 12 and output matching cavity lo with coaxial input and output cables 14 and 18, respectively. An envelope 20, which provides an evacuated chamber of the travelingwave tube 10, consists of along cylindrical structure which'has an enlarged portion 19 at the left extremity thereof, as illustrated in the drawing.

Within the enlarged portion 19 at the left extremity of the envelope 2f), an electron gun 22 is `shown comprising a cathode 24 which is provided with a filament 26, a focusing electrode 28 andan accelerating anode 30 having an 'aperture 31. Focusing electrode 28 has a lfrusto-conical internal surface disposed `at an acute angle from its axis of revolution. A filament source of potential 32 is connected across the filamen-t 26, the positive side of the filament 26 being connected to the cathode 24. Cathode 24 is maintained lat ground potential and focusing electrode 28 is maintained at a suitable potential negative with respect to groundl by means of a biasing source of potential 34. Electrons emitted from the cathode 24 are accelerated from the cathode by Ian accelerating source 36 which is connected between ground andy accelerating `anode 30.

Proceeding along from the gun 22 in the direction of electron fiow produced thereby, there are positioned successively about the path of 4the stream, a matching ferrule 38 `connected by a lead 40 to la helix 42 which is, in turn, connected by a lead 44 to a matching ferrule 46. A collector electrode 48 is positioned at the end of the electron stream path so: as to intercept or collect the stream electrons.

Helix 42 which serves as a-slow-wave circuit for the traveling-Wave tube 10 is fabricated from a material such as tungsten so that it retains its form especially with respect to they ratio of its pitch to its diameter. Ferrule 38, lead 40, helix 42, lead 44, and ferrule 46 are all maintained at the same potential by means of an appropriate connection 50 from ferrule 38 to accelerating anode 30. The magnitude of thelvoltage supplied by accelerating source 36 thus determines the velocity of the stream flowing through the helix 42. This voltage may be of the order yof 1000 volts with respect to ground. The collector electrode 48 is maintained at the same potential Ias accelerating anode 30 by means of a connection to the positive terminal of accelerating source 36.

A solenoid 52 is axially positioned symmetrically about the complete length of the envelope 20. A direct current is maintained in the solenoid 52 by means of a potential source, such as a battery 54, so as to produce an axial magnetic field of uniform density along the length of the tube of the order of 1000 gauss.

In the operation of the amplifier 10 an input signal to be amplified is applied through input coaxialcable 14 to input cavity 12 to launch a traveling-wave along the helix 42., Interactionbetween the electron stream and this travelingv wave results in a transfer of energy from the stream to the vwave which causes it to grow or increase in amplitude. At the end of the helixv 42 the amplified electromagnetic wave n'fiowing along the lead 44 connecting helix 42 to ferrule 46 excitesvan electric field in output matching cavity'16 Ato provide an amplified voutput signal which is available through output coaxial cable 13.

In order to operate the traveling-wave tube 10 at optipower and efiiciency,` it is necessary to project the electron stream developed by theagun 22 as close yto the helix 42 as possible. A rippling in an electron stream as shown by theory and experiment occurs with the use of an axial magnetic field such as that produced by the solenoid 52. This rippling manifests itself as a plurality of stationary periodic diameter Variations. In order to maintain the stream in Fig. l as near the helix 42 as possible, these variations may be minimized by converging the stream from the cathode 24 to a suitable diameter at a point beyond the anode aperture, the rate of change of the stream radius, r, which respect to distance being zero zo is the axial position of the electron gun anode;

rmx is the maximum stream radius;

ro is the stream entrance radius;

z is the axial distance from a suitable reference plane;

0 is the stream entrance angle measured from the line x is the tangent of the entrance angle 0;

x=tan 0:

uf, being the electron stream velocity; Run lll D Pu R0 being space charge density, P0 being theI permittivity of free space; and n being the electron charge to mass ratio, e/m; and

nBg

B0 being the axial magnetic ux density in the stream.

lt is evident from Fig. 2 that the condition x=0 is the best for reducing the radial stream variations. In order to design the electron gun 22 properly, the trajectories of the outer electrons in the stream must therefore belexamined. Referring to Fig. 3, the cathode radius is indicated `by rc. The distance from the cathode 24 to the anode 30 isrepresented by L. The diameter of the anode aperture 31 is denoted by D4 The planes z=-z0 and z=lz0 indicate the planes along the stream where electron motion changes from linear to curvilinear motion and from curvilinear to lineal' motion, respectively.

ln order to determine the correct angle at which t0 converge the stream, a slope equation is developed in the following approximate analysis. By definition the axial component of stream velocity, uz, is given in terms of the voltage of anode 30, V0, as

uz=(2nV)1/2 (1) The radial component of ve1ocity,.u is. the time integral of acceleration or where -l-to and to correspond respectively to +20 and -zm the extent of the lens field, in time;

un is the absolute value of the radial velocity at z=z0 and E, is the radial electric held intensity.

It can reasonably be assumed that the axial electric field intensity, EZ, on the right size of the anode 30 is substantially zero since the collector 48 is at the same potential as the anode. Space charge can also be neglected in most practical cases. Using EZ as the axial electric held intensity at the anode 30, according to Gauss law the following being obtained Substituting Equations 1 and 3 into Equation 4 and noting that u,=0 at z=-lz0 url-TOE.

u-4Vutan 0 (5) which is the tangent of the angle, 0, of convergence as indicated. According to the diode equation, for space charge limited electron ow, the field intensity at the anode Ez, is given by To tan 6-3L (6) The relationship in Equation 6 should therefore be maintained when the stream convergence is substantially linear. This can be accomplished by properly shaping focusing electrode 28 and by maintaining it at an appropriate potential. The exact shape of the focusing electrode can be determined in individual cases with the use of an electrolytic tank by methods well known in the art. Equation 6 is a paraxial ray equation, i. e. Where u, 'uz, in a thin electrostatic lens, i. e. where zD D, which is obviously provided with the anode aperture 31. zo should obviously be smaller than L. i

In order to maintain the confined ilow condition the total flux linking the stream should be maintained constant. If the stream converges substantially linearly, then ode, respectively. According to the geometry of the In using Equation 8 the flux density should then be increased linearly from the cathode to z=z.

The invention is -still practicable even if ro is not small in comparison to L. Equation 8 will then not be quite the correct ratio, but must be changed accordingly. The ratio given by Equation 7 should be changed likewise.

What is claimed is:

In a traveling-wave tube having an electron gun for producing an electron stream having a. substantially uniform diameter, said electron gun comprising a cylindrical thermionic cathode, a focusing electrode having a frustoconical internal surface of revolution disposed concentrically about said cathode and having a shape to develop a substantially linearly converging electron stream from said cathode, an apertured disc electrode disposed coaxially with said cathode and spaced axially therefrom, the distance between said cathode and said disc electrode being equal to L, means for maintaining said disc electrode at a potential to accelerate the electrons emitted at said cathode through the aperture of said disc electrode, means for maintaining said focusing electrode at a potential to cause said stream to converge to a predetermined radius, ro, at a plane where the rate of change of the radius of said stream with respect to the distance along said stream equals zero after the stream emerges from the anode aperture, said stream being converged substantial- -ly linearly at an angle given approximately by and means for maintaining an axial magnetic field within said stream, said field having a linear variation from said cathode to said disc electrode given by the following relationship:

References Cited in the le of this patent UNITED STATES PATENTS Hansell Feb. 5, 1952 Pierce May 3, 1955 UNITED STATES PATENT OFFICE Certificate of Correction Patent No. 2,819,424 January 7, 1958 George R. Brewer It is hereby certiecl that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 8, line 10, for which read withlines 19 and 20, for the mathematical expression,

WH read wp w, column 4, line 9, for size read -side-; line 20, for the lower limit on the integral zo read z0.

Signed and sealed this 1st day of April 1958.

Attest: KARL H. AXLINE, ROBERT C. WATSON,

Attest'ng Ooer. e Uwwm'ssz'oner of Patents. 

